Operation of a power circuit of a liquid crystal display (LCD) device is usually controlled by a pulse width modulation (PWM) circuit. However, the moment when the power circuit is connected or disconnected, impulse current or output voltage overshoot may be generated, which easily damages the PWM circuit and other electronic components. Therefore, in a converter of the LCD device or other power system, a soft start circuit is added to avoid over-high input current. Generally, the soft start circuit preferentially controls drive DUTY of power source to control the input current of the power circuit. When voltage of the power circuit is boosted to a determined voltage, the soft start circuit is turned off which turns on a loop circuit to control output of the power circuit. In a switching process of the soft start circuit, voltage of a loop circuit capacitor of the loop circuit is gradually increased from zero. During the time that the voltage of the loop circuit capacitor is increased from zero, output of the PWM is reduced suddenly, and abnormal output voltage may result if the voltage of the loop circuit capacitor is overhigh at this moment.